The present invention provides for a method for the treatment of waste caustic streams from process industries such as refining and petrochemical. More particularly, the present invention provides for a process for treating caustic waste or spent streams containing sulfur compounds with a packed column recycle reactor for the oxidation of the sulfur compounds in the waste stream.
Due to the presence of sulfur compounds in the crude oil, the refined products like gasoline, LPG and diesel fuel contain sulfur compounds including mercaptans and sulfides. These sulfur compounds must be removed from the hydrocarbon products for odor control and to avoid corrosion problems. A common post-refining sulfur-removal method is caustic washing in which the hydrocarbon streams are contacted with concentrated solutions of caustic soda. The caustic soda reacts with hydrogen sulfide to form sodium sulfide and with mercaptans to form sodium mercaptides. The caustic stream loaded with the above compounds is called spent caustic. A typical spent caustic stream from a refinery contains as much as 10 to 170 g/L of sodium sulfide, 100-1000 ppm of mercaptans and traces of phenols and disulfides. The composition of the spent caustic stream from a petrochemical industry is very similar.
It is economically prudent to regenerate and recycle the spent caustic; the sulfur compounds present in the spent caustic must be oxidized and removed for this disulfides. The composition of the spent caustic stream from a petrochemical industry is very similar.
It is economically prudent to regenerate and recycle the spent caustic; the sulfur compounds present in the spent caustic must be oxidized and removed for this purpose. The oxidation could be performed with air or oxygen. During oxidation, the sodium sulfide is converted to thiosulfate or sulfate; however complete oxidation to sulfate is preferred. The mercaptans are more resistant to oxidation as compared to sulfide. It is known in the open literature that in a strong oxidizing environment, mercaptans are oxidized to disulfide and subsequently to sulfonic acid.
The spent caustic is very odorous and the COD is very high. Hence, even in places where recycling of the spent caustic is not practiced, the mercaptans must be destroyed and the sodium sulfide must be oxidized to environmentally acceptable sulfate for caustic neutralization followed by biological treatment and disposal. This invention describes a process with a packed column recycle reactor for the oxidation of the sulfur compounds in the spent caustic stream for its regeneration and recycling or environmentally acceptable disposal.
The present invention provides for a method for oxidizing sulfur compounds in spent caustic from process industries. The improved method comprises contacting an oxygen-containing gas and spent caustic in a column having structured packing by introducing the spent caustic stream and the oxygen-containing gas into the top and the bottom of the column respectively. The partially oxidized spent caustic is withdrawn from the column bottom into a vessel where it is drawn from the bottom of the vessel and recirculated. The gas phase from the column top is directed into the vessel where the gas phase disengages from the liquid phase and is recirculated to the packed column.
The fully-oxidized spent caustic is withdrawn into a reactor vessel. The sodium sulfate present from the oxidation of the sulfur compounds in the packed column is transferred in solution to a reactor vessel where it is reacted with slaked lime to form calcium sulfate and sodium hydroxide.
The calcium sulfate is relatively insoluble and precipitates. The precipitate can be removed to a clarifier with the calcium sulfate removed as a slurry and the supernatant caustic stream recycled and mixed with the make-up solution prior to its use elsewhere, for example, in caustic washing.
As such, the present invention represents an improvement in that destruction of mercaptans is achieved without the use of ozone or peroxide and the disulfides that result from the mercaptan oxidation are fully oxidized to sulfates. With the elimination of the expensive ozone or peroxide polishing step, a lower capital and operating cost results. Additionally, organics such as phenols are partially destroyed which will reduce the load on subsequent biological treatment.
U. S. Pat. No. 5,439,556 teaches a method for oxidizing sodium sulfide present in white liquor utilized in the pulping of wood to sodium sulfate. An oxygen-containing gas and the white liquor are contacted in a column where oxygen and sodium sulfide react to form an oxidized white liquor which is withdrawn from the bottom of the column. The present invention improves on this process in that the concentration of sulfide in process streams is four to five times higher than in white liquor streams. Mercaptans are also present in process streams as well as other compounds such as phenols. Lastly, the ""556 patent teaches that there is heat generated by the exothermic oxygen/sodium sulfide reaction. The inventive process will generate more heat due to the higher sulfide and mercaptan concentrations in process streams.